Inorganic hydrophilic coating solution, hydrophilic coating film obtained therefrom, and member using same

ABSTRACT

An inorganic hydrophilic coating solution including (a) an aqueous solution containing an amorphous silicate compound obtained by hydrolyzing and condensing a tetrafunctional silicon compound having a purity of 99.0 mass % or greater in an aqueous medium in the presence of a basic compound at a temperature within a range from normal temperature to 170° C., (b) water, and optionally, (c) not more than 30 mass % of an alcohol, a ketone, or a surfactant, where the concentration of the solid fraction derived from the aqueous solution containing the amorphous silicate compound is 0.01 to 2.0 mass % and the pH is 5 to 8; an inorganic hydrophilic coating film formed from a dried and cured product of the inorganic hydrophilic coating solution; a member having a substrate and the inorganic hydrophilic coating film formed on the surface of the substrate; and a cover panel for a solar cell module including the member.

TECHNICAL FIELD

The present invention relates to a hydrophilic coating solution forprotecting the surfaces of various substrates from staining. Thehydrophilic coating solution of the present invention is an inorganichydrophilic coating solution, yet can form a film that can suppresslight reflection and coherence, and has a substrate stain resistancefunction derived from the extremely high hydrophilicity. Moreover, thecoating solution is stable in aqueous systems and neutral environments,and can form a coating film even by curing at normal temperature.Accordingly, handling of the solution is safe and easy, during bothindustrial inline production and onsite operations, meaning a strong andtransparent hydrophilic coating can be formed easily in any location.

BACKGROUND ART

A variety of stain-resistant coating agents have long been availablecommercially, primarily for residential construction materials such asglass and exterior walls.

Previously, techniques in which water repellency was used to repel theactual contaminants were widely employed. However, in recent years, ithas become increasingly obvious that as a result of long-term exposureof these types of water-repellent films, a problem has arisen in thatthe water repellency of the material has become a source ofelectrostatic charging, which actually promotes the adsorption of dustand the adhesion of soiling.

In light of these circumstances, a variety of hydrophilic coatingsolutions have begun to be tested, with the expectation of preventingelectrostatic charging and enabling soiling to be washed away byrainfall, and various methods of hydrophilizing members are beinginvestigated, mainly focused on photocatalytic materials. In particular,photocatalysts are not only hydrophilic upon irradiation with light, butcan also be expected to exhibit an oxidative decomposition action onsoiling, and there are many practical examples of such photocatalysts.

For example, applications to organic materials including plasticmaterials and applications to decorative materials have been activelyinvestigated in recent years, mainly in relation to applications toexterior tiles, glass, exterior wall coatings, internal filters of aircleaners, and inorganic substrates (such as ceramics and metals) (seePatent Document 1 and Patent Document 2).

However, currently, all of the hydrophilic coating solutions that havebeen marketed and developed in the manner described above using numerouscomponents suffer from some form of problem.

Specifically, hydrophilic coating agents containing a water-solublepolymer as the main component have poor durability, and as the curingreaction of the coating film progresses, the hydrophilic active groupssuch as hydroxyl groups are gradually eliminated.

Inorganic coating agents such as silicones exhibit excellent durabilityand hydrophilicity, but the coating solution itself is often an organicsolvent-based system, which can sometimes limit the working conditions.Even solventless systems suffer from small amounts of volatilization ofvolatile oligomers, meaning the working conditions are limited.Particularly in the case of coating agents having high curingreactivity, storage problems also arise due to the short lifespan of thesolution.

In coating solutions containing an added photocatalyst, because thephotocatalyst itself is a powder having extremely high cohesion, thecoating solution must be formed as a coarsely dispersed sol (dispersedparticle size: 100 nm to several wn) to which any of the various polymervehicles has been added, and therefore the external appearance followingcoating may sometimes develop cloudiness, resulting in inferiortransparency. Further, the hydrophilicity of the coating film can onlymanifest in environments where light is present. Moreover, decompositionand degradative dislodgement of the coating film caused by decompositionof the vehicle by photocatalytic reactions is also unavoidable.

In this manner, there is no coating solution currently available whichis safe and easy to work with, and can form a film capable of exhibitinga satisfactory hydrophilic action over a long period of time.

Further, these coating agents often contain an added metal oxide such assilica containing a large number of hydroxyl groups in order to impartgood hydrophilicity, but particles of these metal oxides have a highrefractive index, and when coated onto a transparent substrate, andparticularly a substrate having a low refractive index such as glass orpolycarbonate, tend to display high reflectance relative to diagonallyincident light striking the coated surface at an angle other than thenormal direction, meaning the external appearance is often impaired.

Accordingly, there is considerable demand for a technique capable ofsimultaneously addressing the problems facing existing hydrophiliccoating solutions, namely problems such as:

(1) difficulty in achieving high hydrophilicity,(2) inability to maintain hydrophilicity for long periods,(3) low transparency for the solution itself or the formed coating film,meaning application to substrates other than white substrates isdifficult,(4) limits on the pot life and/or the coating properties of the solutionitself, and(5) severe reflection from the coating film, thus impairing the externalappearance, especially when viewed from an angle.

[Patent Document 1] JP 2006-116461 A

[Patent Document 2] JP 2006-272757 A

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

The present invention has been developed in light of the above problems,and has an object of providing a low-reflection, low-coherence inorganichydrophilic coating solution which can effectively resolve theaforementioned problems (1) to (5), and yields a coating film whichcombines high hydrophilicity and high transparency, as well as providinga hydrophilic coating film obtained from the coating solution and amember using the coating film.

Means to Solve the Problems

As a result of intensive investigations aimed at achieving the aboveobject, the inventors of the present invention were able to complete thepresent invention.

In other words, a first aspect of the present invention provides aninorganic hydrophilic coating solution comprising:

(a) an amorphous silicate compound-containing aqueous solution obtainedby hydrolyzing and condensing a tetrafunctional silicon compound havinga purity of 99.0 mass % or greater in an aqueous medium in the presenceof a basic compound at a temperature within a range from normaltemperature to 170° C.,

(b) water, and

(c) in some cases, not more than 30 mass % of an alcohol, a ketone, asurfactant, or a mixture of 2 or more thereof, wherein

the concentration of the solid fraction derived from the amorphoussilicate compound-containing aqueous solution is 0.01 to 2.0 mass %, andthe pH is 5 to 8.

For the component (c), examples of the alcohol include methanol,ethanol, n-propanol, isopropanol, n-butanol and isobutanol. Examples ofthe ketone include acetone, diacetone alcohol, methyl ethyl ketone,diisobutyl ketone, methyl isobutyl ketone and cyclohexanone. Examples ofthe surfactant include anionic surfactants such as soap (sodium fattyacid)-based surfactants, alkylbenzenesulfonate (ABS or LAS)-basedsurfactants, higher alcohol sulfate ester (AS)-based surfactants,polyoxyethylene alkyl ether sulfate (AES)-based surfactants, a-sulfofatty acid ester (a-SF)-based surfactants, α-olefin sulfonate-basedsurfactants, monoalkyl phosphate ester (MAP)-based surfactants, andalkane sulfonate (SAS)-based surfactants; cationic surfactants such asalkyltrimethylarnmonium salt-based surfactants, dialkyldimethylammoniumsalt-based surfactants, alkyldimethylbenzylammonium salt-basedsurfactants, and amine salt-based surfactants; amphoteric surfactantssuch as alkylamino fatty acid salt-based surfactants, alkylbetaine-basedsurfactants, and alkylamine oxide-based surfactants; and nonionicsurfactants such as polyoxyethylene alkyl ether (AE)-based surfactants,polyoxyethylene alkylphenol ether-based surfactants, alkylglucoside-based surfactants, polyoxyethylene fatty acid ester-basedsurfactants, sucrose fatty acid ester-based surfactants, sorbitan fattyacid ester-based surfactants, polyoxyethylene sorbitan fatty acidester-based surfactants, fatty acid alkanolamide-based surfactants,surfactants in which the hydrophobic groups of these nonionicsurfactants have been modified with silicone or fluorine, acetylenealcohol-based surfactants, and acetylene glycol-based surfactants.

The amount of the component (c) is not more than 30 mass % of thehydrophilic coating solution of the present invention, namely 0 to 30mass %, and is preferably 0 to 20 mass %, and more preferably 0 to 10mass %.

The contact angle with water of a coating film formed from a dried andcured product of the inorganic hydrophilic coating solution is not morethan 20°, and is preferably 15° or less.

In a preferred embodiment of the present invention, the basic compoundin the component (a) is an organic ammonium salt, an alkylamine, analkanolamine, a nitrogen-containing heterocyclic compound, or acombination of 2 or more thereof, and the amount added of the basiccompound is 100 mol % or more relative to the tetrafunctional siliconcompound.

In another preferred embodiment of the present invention, thetetrafunctional silicon compound is:

an amorphous silica having an SiO₂ content of 99 mass % or greater, andpreferably 99.0 mass % or greater, a content of each of Na₂O, K₂O,Fe₂O₃, CaO, SO₃, MgO and P₂O₅ of 0.1 mass % or less, and a primaryparticle size of 500 nm or less,

a tetrafunctional silicon alkoxide compound such as tetraethoxysilane(ethyl orthosilicate) having a purity of 99.0 mass % or greater,

a tetrafunctional silicon halide compound such as silicon tetrachloridehaving a purity of 99.0 mass % or greater, or

a combination of 2 or more thereof.

In a spectrum obtained by performing a ²⁹Si-NMR measurement of theamorphous silicate compound-containing aqueous solution of component(a), when the surface area ratio of a peak attributable to a Q_(n)structure (wherein n is an integer of 0 to 4) relative to all of thepeaks is used to calculate a molar ratio R_(n) (wherein n is as definedabove) of silicon atoms within the Q_(n) structure relative to all ofthe silicon atoms within the amorphous silicate compound-containingaqueous solution, then it is preferable that the following relationshipsare satisfied:

(R₀+R₁+R₂+R₃)≧90 mol %, R₃≧40 mol %, and R₄≦5 mol %.

The inorganic hydrophilic coating solution of the present inventionpreferably further comprises, as a photocatalyst: (d) at least one typeof microparticles, selected from the group consisting of metal oxideparticles, metal chalcogenide particles and organometallic complexparticles, having n-type semiconductor properties, and having a primaryparticle size of 1 to 100 nm. The microparticles are preferably at leastone type of microparticles selected from the group consisting oftitanium dioxide particles and tungsten trioxide particles, and having aprimary particle size of 1 to 100 nm. At least one metal selected fromthe group consisting of vanadium, manganese, iron, cobalt, nickel,copper, zinc, niobium, molybdenum, ruthenium, rhodium, palladium,silver, tin, tungsten, platinum and gold, a compound (such as an oxide)of one of these metals, or a combination thereof is preferably supportedon the aforementioned microparticles. In the present description, theexpression “a primary particle size of 1 to 100 nm” means that whenapproximately 10 fields of view each 10 p.m square are observed using atransmission electron microscope, the particle sizes of all visibleparticles are within a range from 1 to 100 nm.

A second aspect of the present invention provides a hydrophilic coatingfilm formed from a dried and cured product of the above inorganichydrophilic coating solution.

The hydrophilic coating film has a contact angle with water of not morethan 20°, and preferably 15° or less, and the contact angle with waterfollowing storage in a dark place for one month is preferably maintainedat not more than 20°, and more preferably 15° or less.

A third aspect of the present invention provides a member having asubstrate, and the aforementioned hydrophilic coating film provided on asurface of the substrate.

The substrate is preferably a glass-based substrate, apolycarbonate-based substrate, an acrylic-based substrate, apolyester-based substrate or a fluorine-based substrate. It ispreferable that when the surface on which the hydrophilic coating filmis provided is observed at any angle from the normal direction relativeto the surface through to the tangential direction of the surface,neither interference color nor cloudiness anomalies can be visuallydetected.

A fourth aspect of the present invention provides a cover panel for asolar cell module comprising the above member.

Effects of the Invention

The low-reflection, low-coherence inorganic hydrophilic coating solutionof the present invention can use an aqueous medium (namely, either wateron its own, or a combination of water with at least one of an alcohol, aketone and a surfactant), and therefore a coating solution that is safeand causes no substrate damage can be formed, and low-temperature curingat a temperature of about normal temperature to 100° C. is possible. Theobtained coating film exhibits excellent levels of transparency andhardness, as well as excellent removability of adhered soiling, thesurface maintains its hydrophilicity even after one month, and thehydrophilicity continues to manifest even in dark places. The formedcoating film is composed entirely of inorganic substances and thereforeexhibits good strength, and despite being an inorganic coating film,still exhibits superior transparency to incident light from alldirections.

Moreover, the coating solution has a pH close to neutral and iswater-based, and can therefore be used regardless of the workenvironment or method. Because the coating solution has these liquidproperties and yet also has a liquid lifespan of half a year or longer,it offers excellent workability.

Accordingly, the present invention can provide a low-reflection,low-coherence inorganic hydrophilic coating solution which has excellentperformance and handling properties, and provides excellent maintenanceof aesthetic features.

Best Mode of Carrying Out the Invention

The present invention is described below in more detail. In the presentdescription, “normal temperature” refers to a temperature of 10 to 30°C.

The low-reflection, low-coherence inorganic hydrophilic coating solutionaccording to the present invention contains a silicate having a specificcomposition and content ratio as the main component, and is obtained byproperly adding a stain-resistant material such as a photocatalyst.

[Tetrafunctional Silicon Compound]

As the tetrafunctional silicon compound which functions as the silicateraw material, can be use any conventionally known material, providedthat the purity is 99.0 mass % or greater. A single tetrafunctionalsilicon compound may be used alone, or a combination of 2 or morecompounds may be used. Examples of the tetrafunctional silicon compoundinclude silica, tetrafunctional silicon alkoxide compounds andtetrafunctional silicon halide compounds. In the present description, a“tetrafunctional silicon compound” refers to a compound which, uponhydrolysis and condensation, forms a silicate compound having astructure represented by the following formula:

a tetrafunctional silicon alkoxide compound describes a silicon alkoxidecompound which yields a structure represented by the above formula uponhydrolysis and condensation, and a tetrafunctional silicon halidecompound describes a silicon halide compound which yields a structurerepresented by the above formula upon hydrolysis and condensation.

Silica

Examples of the types of silica which can be used include fumed silica,sol-gel method silica, solution-dispersed colloidal silica, and silicaextracted from rice husks. A single silica may be used alone, or acombination of 2 or more varieties of silica may be used. Among thevarious possibilities, an amorphous silica having an SiO₂ content of99.0 mass % or greater, a content of each of Na₂O, K₂O, Fe₂O₃, CaO (freecalcium oxide), SO₃, MgO and P₂O₅ of 0.1 mass % or less, and a primaryparticle size of 500 nm or less is preferable.

If the amount of an alkali impurity containing Na, K, or Ca or the like(a compound containing an alkali metal or an alkaline earth metal), or aS- or P-based impurity exceeds 1 mass %, then the stability of thecoating solution obtained following dilution and pH adjustment tends todeteriorate, and may be prone to gelling or the precipitation of solidcontaminants. Further, particularly in the case where a titaniumdioxide-based photocatalyst is added, if residual Na⁺ ions exist, thenthese are gradually incorporated within the TiO₂, generating sodiumtitanate (NaTiO₃) that is inactive relative to light, thus inhibitingphotocatalytic reactions. For this reason, Na-containing products arenot desirable.

Further, if the amount of a metal impurity of Fe or Al or the likeexceeds 0.1 mass %, then the solution may become colored, and therefractive index may increase. Moreover, the transparency of the film,and particularly the transparency relative to light that is diagonallyincident on the surface of the formed coating film, may deteriorate.

In the present description, “a primary particle size of 500 nm or less”means that when approximately 10 fields of view each 10 μm square areobserved using a transmission electron microscope, no particlesexceeding 500 nm are observed.

Tetrafunctional Silicon Alkoxide Compound

Examples of the tetrafunctional silicon alkoxide compound includesilicon alkoxide compounds represented by structural formula (1) shownbelow:

Si(OR¹)_(x)(OH)_(4-x)   (1)

wherein R¹ represents an organic group, and x represents an integer of 1to 4, and condensation products thereof. A single tetrafunctionalsilicon alkoxide compound may be used alone, or a combination of 2 ormore compounds may be used. Further, the tetrafunctional siliconalkoxide compound may be used in combination with a silica.

Multiple 12¹ groups in the above formula may be the same or different.Specific examples of R¹ include alkyl groups such as a methyl group(CH₃), ethyl group (CH₂CH₃), propyl group (CH₂CH₂CH₃), isopropyl group(CH₂(CH₃)CH₃) and butyl group (CH₂CH₂CH₂CH₃); and alkoxysilyl groupssuch as a triethyoxysilyl group (Si(OCH₂CH₃)₃). Among the variouspossibilities, tetramethoxysilane (in which x represents 4 and all theR¹ groups are methyl groups) and tetraethoxysilane (in which xrepresents 4 and all the R¹ groups are ethyl groups) can be usedparticularly favorably.

Tetrafunctional Silicon Halide Compound

An example of the tetrafunctional silicon halide compound is silicontetrachloride.

[Basic Compound]

Examples of the basic compound used in the hydrolysis and condensationinclude organic bases such as organic ammonium salts, alkylamines,alkanolamines, nitrogen-containing heterocyclic compounds, andcombinations of 2 or more of these bases. A single basic compound may beused alone, or a combination of 2 or more basic compounds may be used.

Organic Ammonium Salt

Examples of the organic ammonium salt include salts represented bystructural formula (2) shown below:

R² ₄N⁺X⁻  (2)

wherein R² represents an organic group, and X represents a hydroxylgroup (OH) or a halogen atom (F, Cl, Br or I).

A single organic ammonium salt may be used alone, or a combination of 2or more organic ammonium salts may be used.

The R² groups in the above formula may be the same or different.Specific examples of R² include alkyl groups such as a methyl group(CH₃), ethyl group (CH₂CH₃), propyl group (CH₂CH₂CH₃), isopropyl group(CH₂(CH₃)CH₃) and butyl group (CH₂CH₂CH₂CH₃); and hydroxyalkyl groupssuch as a methylol group (CH₂OH) and an ethylol group (CH₂CH₂OH). Amongthe various possibilities, tetramethylammonium hydroxide andtetrabutylammonium hydroxide can be used particularly favorably.

Alkylamine

Examples of the alkylamine include nitrogen-containing compoundsrepresented by structural formula (3) shown below:

R³ _(y)—NH_(3-y)   (3)

wherein R³ represents an alkyl group, and y represents an integer of 1to 3, and salts thereof. A single alkylamine may be used alone, or acombination of 2 or more alkylamines may be used.

Multiple R³ groups in the above formula may be the same or different.Specific examples of R³ include alkyl groups such as a methyl group(CH₃), ethyl group (CH₂CH₃), propyl group (CH₂CH₂CH₃), isopropyl group(CH₂(CH₃)CH₃) and butyl group (CH₂CH₂CH₂CH₃).

Alkanolamine

Examples of the alkanolamine include nitrogen-containing compoundsrepresented by structural formula (4) shown below:

R⁴ _(y)—NH_(3-y)   (4)

wherein R⁴ represents a hydroxyalkyl group, and y is the same as definedabove, and salts thereof. A single alkanolamine may be used alone, or acombination of 2 or more alkylamines may be used.

Multiple R⁴ groups in the above formula may be the same or different.Specific examples of R⁴ include hydroxyalkyl groups such as a methylolgroup (CH₂OH) and an ethylol group (CH₂CH₂OH).

Aqueous Medium

The aqueous medium used in preparing the component (a) refers to amedium composed only of water, or composed of a mixture of water andanother medium, for example at least one alcohol or ketone, wherein thetotal amount of media other than water within the mixture is not morethan 30 mass %, and preferably 20 mass % or less.

[Conditions for Hydrolysis and Condensation]

The hydrolysis and condensation of the tetrafunctional silicon compoundis performed in an aqueous medium, in the presence of a basic compound,at a temperature within a range from normal temperature to 170° C.,using the aforementioned tetrafunctional silicon compound as a rawmaterial in an amount that preferably represents 1 to 20 mass %, andmore preferably 1 to 15 mass %, of the total mass of the reaction system(the raw material mixed liquid). The amount added of the basic compoundis preferably at least 100 mol %, and more preferably 100 to 500 mol %,relative to the amount of the tetrafunctional silicon compound.

When the tetrafunctional silicon compound is a silica, a raw materialmixed liquid is prepared in which the SiO₂ concentration is 1 to 15 mass%, the basic compound concentration is 2 to 25 mass % and the remainderis water, and this raw material mixed liquid is then preferably stirredunder heating at 80 to 130° C. This yields a transparent hydrolyzedsolution. When hydrolysis and condensation of the silica is performedunder these conditions, the hydrolysis reaction proceeds readily, andtherefore residual insoluble silica is unlikely, and becausedecomposition of the added basic compound is unlikely to occur, theprecipitation of solid matter is also unlikely. Moreover, the basiccompound used in the hydrolysis is preferably an organic ammonium salt,and of such salts, tetramethylammonium hydroxide or tetrabutylammoniumhydroxide can be used particularly favorably.

If an inorganic base such as sodium hydroxide is used, then thepolycondensation reaction of the obtained silicate tends to proceedrapidly, making it difficult to obtain a silicate of the desiredcomposition, and the lifespan of the solution also shortens. Further,even if ions are removed in post-processing, residual cations tends tobe retained in the film for long periods, causing deterioration in thefilm properties.

In a spectrum obtained by performing a ²⁹Si-NMR measurement of theaqueous solution containing the amorphous silicate compound obtained bythe hydrolysis and condensation, the chemical shift of each of the peaksis attributed as follows.

Q₀: vicinity of −71.5 ppm 0 condensate silicate monomer Q₁: vicinity of−79 to −83 ppm monocondensate Q₂: vicinity of −87 to −91 ppmdicondensate Q₃: vicinity of −96 to −100 ppm tricondensate Q₄: vicinityof −107 to −112 ppm tetracondensate

As described above, it is preferable that the relationships(R₀+R₁+R₂+R₃)≧90 mol %, R₃≧40 mol %, and R₄≦5 mol % are satisfied.Provided that the relationship R₄ <5 mol % is satisfied, the amount ofthe low-polarity Q₄ structure does not become too large, and thereforethe adhesion of the cured film tends to improve. Further, provided thatthe relationship R₃>40 mol % is satisfied, the amount of the Q₃structure is satisfactory, and therefore the condensation reactivity dueto the Q₀ to Q₂ structures is prevented from becoming too high, thusmaking the stability of the inorganic hydrophilic coating solution morefavorable.

The “inorganic hydrophilic coating solution” of the present inventiondescribes a coating solution which, upon removal of the solvent bydrying and curing, yields a substantially inorganic hydrophilic curedproduct. Here, the expression “substantially inorganic” means that theamount of inorganic matter relative to the total mass of the curedproduct is preferably at least 60 mass %, and more preferably 80 mass %or greater.

[Preparation and Application of Coating Solution]

The aqueous solution containing the amorphous silicate compound preparedby performing hydrolysis and condensation under the conditions describedabove is diluted so that the solid fraction concentration is within arange from 0.01 to 2.0 mass %, and the cations of the basic compoundadded to effect the hydrolysis and condensation are removed, for exampleby an ion exchange resin, to adjust the pH to a value of 5 to 8, thusobtaining the coating solution of the present invention. By adjustingthe solid fraction concentration to 0.01 to 2.0 mass %, and preferablyto a value within a range from 0.1 to 1.0 mass %, the stability of theobtained solution tends to be more favorable, and precipitation of solidmatter is unlikely.

[Addition of Photocatalyst]

When a photocatalyst of the component (d) is also added to the inorganichydrophilic coating solution of the present invention, the concentrationof the added photocatalyst is preferably within a range from 0.01 to 10mass %, and more preferably from 0.1 to 5 mass %. When the concentrationof the photocatalyst satisfies this range, the stain-resistant activityprovided by the photocatalyst tends to be more favorable, and thetransparency is more readily maintained, meaning the external appearancetends to be more favorable.

Further, when the concentration (mass %) of the silicate compound withinthe inorganic hydrophilic coating solution is termed A, and theconcentration (mass %) of the photocatalyst is termed B, then the ratioof A: (A+B) is preferably within a range from 0.05:99.5 to 99.5:0.05,and more preferably from 99.5:0.05 to 70:30. When the value of A: (A+B)satisfies this range, the stain-resistant activity due to the oxidativedegradation derived from the photocatalyst manifests more readily, andthe strength and transparency of the film tend to be favorable.

[Formation of Coating Film]

There are no particular limitations on the substrate to which theinorganic hydrophilic coating solution of the present invention isapplied, provided that a coating film is able to be formed. Examples ofthe substrate material include inorganic materials and organicmaterials, wherein the inorganic materials include both non-metallicinorganic materials and metallic inorganic materials. These substratematerials may have all manner of shapes, depending on the purpose andthe application.

Examples of the non-metallic inorganic materials include glass andceramic materials. These materials can be in the form of commercializedproducts such as tiles, vitreous products, and mirrors.

Examples of the metallic inorganic materials include cast iron, steel,iron, iron alloys, aluminum, aluminum alloys, nickel, nickel alloys andzinc die casts, and these materials may be plated, or coated with anorganic coating material. Further, a metal plating film may be formed onthe surface of a non-metallic inorganic material or an organic material.

Examples of the organic material include synthetic resin materials suchas vinyl chloride resin, polyethylene, polypropylene, polycarbonate,acrylic resin polyacetal, fluororesin, silicone resin, ethylene-vinylacetate copolymer (EVA), acrylonitrile-butadiene rubber (NBR),polyethylene terephthalate (PET), polyethylene naphthalate (PEN),polyvinyl butyral (PVB), ethylene-vinyl alcohol copolymer (EVOH),polyimide, polyphenylene sulfide (PPS), polyetherimide (PEI),polyetheretherimide (PEED, polyetheretherketone (PEEK), melamine resin,and acrylonitrile-butadiene-styrene (ABS) resin; and natural, syntheticor semi-synthetic fibrous materials and fibrous products. Thesematerials may be commercialized products of the desired shape andcomposition, including films, sheets, other molded items, and laminates.

Any conventional method can be used to apply the inorganic hydrophiliccoating solution of the present invention to a substrate. Specifically,a method such as a dip coating method, spin coating method, spraycoating method, roller method, brush application method, impregnationmethod, roll method, wire bar method, die coating method, gravureprinting method or inkjet printing method can be used to form a coatingfilm on the substrate.

The thickness of the formed coating film is preferably within a rangefrom 1 to 500 nm, and a thickness within a range from 50 to 300 nm isparticularly desirable. Provided that the thickness is within the rangefrom 1 to 500 nm, the coating film is more likely to exhibit favorablestrength and transparency, and be resistant to cracking.

Drying and curing of the coating film formed from the inorganichydrophilic coating solution of the present invention may be performedby standing at normal temperature or by heating. When heating isconducted, the heat treatment is preferably performed within atemperature range from 50 to 200° C. for a period of 1 to 120 minutes,and is more preferably performed within a temperature range from 60 to110° C. for a period of 5 to 60 minutes.

The water contact angle of the coating film formed by application of theinorganic hydrophilic coating solution of the present invention istypically not more than 20°, and is preferably 15° or less. Providedthat the water contact angle is 20° or less, favorable stain resistanceproperties can be more readily achieved.

Further, the transparency of the coating film formed by application ofthe inorganic hydrophilic coating solution of the present invention ispreferably of a level such that, relative to the transparency of theoriginal substrate, the reduction in total light transmittance followingformation of the coating film is not more than Δ3%, and the increase inthe haze is not more than Δ2%. Provided that the reduction in totallight transmittance due to the coating film is not more than Δ3%, thetransparency can be readily maintained, and the external appearancetends to be favorable. Further, provided that the increase in the hazeis not more than Δ2%, cloudiness is unlikely to occur in the film, andthe external appearance tends to be favorable.

EXAMPLES

Specifics of the present invention are described below based on a seriesof examples and comparative examples. However, the present invention isin no way limited by these examples.

Example 1

For the tetrafunctional silicon compound, a high-purity fumed silica(not a commercially available product, an amorphous silica manufacturedby Shin-Etsu Chemical Co., Ltd. having a primary particle size of notmore than 500 nm) having the properties shown in the table below (units:mass %) was used.

TABLE 1 SiO₂ Fe₂O₃ Total carbon MgO SO₃ Na₂O K₂O CaO P₂O₅ Cl⁻ 99.04<0.01 0.01 <0.01 0.03 <0.01 <0.01 0.10 0.008 <0.01

The above fumed silica, water, and tetramethylammonium hydroxide (25mass % aqueous solution, manufactured by Toyo Gosei Co., Ltd.) weremixed so that the ratio of silica : tetramethylammonium hydroxide=1:2(molar ratio) and the silica content of the system at the start of thereaction was 5 mass %, and the mixture was then stirred under heating at110° C. for 2 hours. The resulting solution was diluted with water toobtain a solid fraction concentration of 0.5 mass %, and was thenadjusted to a pH of 7.0 using an ion exchange resin (product name: Dowex50W-X8, manufactured by Dow Coming Corporation), thus obtaining acoating solution (solid fraction concentration: 0.5 mass %).

Example 2

Tetraethyl orthosilicate (purity: 99.9 mass % or higher, manufactured byTama Chemicals Corporation, hereafter sometimes referred to as TEOS),water, tetramethylammonium hydroxide (25 mass % aqueous solution,manufactured by Toyo Gosei Co., Ltd.) and acetone (manufactured by WakoPure Chemical Industries, Ltd., special grade) were mixed so that theratio of tetramethylammonium hydroxide: tetraethyl orthosilicate=4:1(molar ratio), and so that at the start of reaction, the tetraethylorthosilicate content of the system was 15 mass % and the acetonecontent was 42 mass %, and the mixture was then stirred at normaltemperature for 5 hours, yielding a white precipitate. The solvent wasremoved from the system, the precipitate was recovered, and followingwashing with acetone, the precipitate was redissolved in water, dilutedwith water to obtain a solid fraction concentration of 0.5 mass %, andthen adjusted to a pH of 7.0 using an ion exchange resin (product name:Dowex 50W-X8, manufactured by Dow Corning Corporation), thus obtaining acoating solution (solid fraction concentration: 0.5 mass %).

Example 3

A photocatalyst Sagan Coat TO sol (product name: TO-85, a dispersion ofAnatase-type titanium oxide, manufactured by Kon Corporation) was addedto the coating solution of Example 1 in sufficient amount that the ratioof the solid fraction within the coating solution obtained in Example 1:TiO₂=90:10 (mass ratio), thus preparing a coating solution.

Example 4

A photocatalyst Sagan Coat TO sol (product name: TO-85, a dispersion ofAnatase-type titanium oxide, manufactured by Kon Corporation) was addedto the coating solution of Example 2 in sufficient amount that the ratioof the solid fraction within the coating solution obtained in Example 2:TiO₂=90:10, thus preparing a coating solution.

Comparative Example 1

With the exception of using a commercially available colloidal silica(purity: 20 mass %, product name: Snowtex OS, manufactured by NissanChemical Industries, Ltd.) instead of the high-purity fumed silica usedin Example 1, a coating solution (solid fraction concentration: 0.5 mass%) was prepared in the same manner as Example 1.

Comparative Example 2

With the exception of using a commercially available silicate hydrolyzedsolution (purity: 10 mass %, product name: HAS-10, manufactured byColcoat Co., Ltd.) instead of the tetraethyl orthosilicate used inExample 2, a coating solution (solid fraction concentration: 0.5 mass %)was prepared in the same manner as Example 2.

Comparative Example 3

A commercially available peroxo titanic acid aqueous solution (productname: PTA-85, manufactured by Kon Corporation) was simply diluted withwater to prepare a coating solution (solid fraction concentration: 0.5mass %).

Comparative Example 4

A commercially available product (product name: TPX-85, manufactured byKon Corporation) composed of a mixture of an aqueous solution of peroxotitanic acid and an Anatase-type titanium oxide was simply diluted withwater to prepare a coating solution (solid fraction concentration: 0.5mass %).

[²⁹ Si-NMR Measurements]

The coating solutions prepared in Examples 1 and 2 and ComparativeExamples 1 and 2 were each subjected to a ²⁹Si-NMR measurement, and ineach of the thus obtained spectra, the surface area ratio of each peakattributable to a Q_(n) structure (wherein n is an integer of 0 to 4)relative to all of the peaks was used to calculate a molar ratio R_(n)(wherein n is as defined above) of silicon atoms within the Q_(n)structure relative to all of the silicon atoms within the coatingsolution. At this time, a previously prepared calibration curveillustrating the relationship between the above surface area ratio andR_(n) was used.

[Application of Coating Solutions]

The coating solution was applied to a substrate formed from a PET(polyethylene terephthalate) film (thickness: 50 μm) that had been cutto A4 size and subjected to a corona discharge treatment, and thecoating solution was then dried by heating to prepare a coating filmhaving a thickness of approximately 200 nm. The conditions for theheated drying involved heating at 70° C. for 30 minutes.

[Normal Temperature Curing Time]

Following formation of the coating film, the film was left to stand atnormal temperature, and a dry Kimwipe was used to scrub the film. Thetime at which this operation no longer resulted in the generation ofvisually detectable blemishes on the coating film was specified as thenormal temperature curing time.

Coating films obtained by performing the normal temperature curingdescribed above were used for the following tests.

[Film Thickness Measurement]

The film thickness was measured using a thin film measurement deviceF-20 (product name, manufactured by Filmetric, Inc.).

[Water Contact Angle]

The water contact angle for the coating film was measured at normaltemperature using a contact angle meter CA-A (product name, manufacturedby Kyowa Interface Science Co., Ltd.). Measurement of the water contactangle was also performed for a coating film that had been left in a darkplace for one month. Moreover, the water contact angle was also measuredfor a coating film that had been exposed outdoors for one month.

[Water Film Formation after One Month Exposure]

A coating film that had been exposed outdoors for one month was broughtinto contact with water, and an evaluation was made, against thefollowing criteria, as to whether or not a water film is formed on thesurface of the coating film.

O: a water film was formed uniformly.

Δ: a water film was formed, but was not uniform.

x: the coating film repelled the water, and a water film was not formed.

[Total Light Transmittance, Haze]

These were measured using a digital haze meter NDH-20D (manufactured byNippon Denshoku Industries Co., Ltd.). The measurement of diagonallyincident light was preformed with the test sample inclined at an angleof 45°.

[Visually Detectable External Appearance Anomalies]

The surface on which the coating film was provided was observed at allangles from the normal direction relative to the surface through to adirection parallel to the surface, and visual confirmation was made asto whether interference color appeared, or whether cloudiness occurreddue to reflection.

[Film Adhesion]

Testing was performed in accordance with the cross-cut tape peeling testof JIS K 5400 8.5, and the number of residual blocks was recorded. Ahigher number indicates superior adhesion.

[Film Strength]

Following preparation of each of the above coating films, each coatingfilm was left to stand at normal temperature for at least 48 hours, andthe coating film was then rubbed with a finger and scrubbed with a dryKimwipe, and evaluated against the following criteria.

O: no blemishes were formed.

x: blemishes were formed.

[Stain Resistance]

The external appearance of a coating film that had been exposed outdoorsfor one month in the manner described above was inspected visually andevaluated against the following criteria.

O: no change in the external appearance from the initial state.

Δ: not stained, but the film itself developed color interference,whitening, or both.

x: water marks, rain spots, or both occurred.

[Optical Properties]

The optical properties of the coating film were evaluated against thefollowing criteria.

O: no detectable whitening or interference color, and no reduction intransparency when tilted.

Δ: whitening, interference color, or both were detected, but noreduction in transparency occurred when tilted.

x: whitening, interference color, or both were detected, and a reductionin transparency occurred when tilted.

The results of the above tests are shown in Table 2.

TABLE 2 Exam- Exam- Compar- Exam- Exam- ple 3 ple 4 ative Compar-Compar- Compar- ple 1 ple 2 S T Example 1 ative ative ative S T method +method + Snowtex Example 2 Example 3 Example 4 method method TiO2 TiO2OS HAS-10 PTA TPX-85 Main component(s) sili- sili- silicate + silicate +silica Sili- peroxo peroxo cate cate photo- photo- cate titanic titaniccatalyst catalyst acid acid + photo- catalyst Compounds used silicatesilicate silica Sili- peroxo peroxo (raw material: (raw material: catetitanic titanic silica) TEOS) acid acid Monomer ΣRn (n = 99 99 27 50 — —condensation 0→3)/mol % degree, molar R3/mol % 53 43 25 40 — — ratioR4/mol % <1 <1 73 10 — — Normal temperature curing time/hr 2 2 5 5  48<1  48<  48< Initial water contact angle/deg 5 6 5 7 12 56 24 22 Watercontact angle after 1 month 11 11 8 9 14 50 21 21 in dark/deg Watercontact angle after 1 month <5 <5 <5 <5  6 45 19 15 exposure/deg Waterfilm formation after 1 month ∘ ∘ ∘ ∘ ∘ x Δ ∘ exposure Total lighttransmittance perpendicular +4.2 +4.0 +3.1 +3.2   3.1 +3.7   +0.6   −3.7(change relative to incident light substrate/Δ % 45° incident +4.2 +4.0+2.9 +3.2   3.7 +1.2   +0.4   −5.1 light Reduction in transparency uponnone None none none none slight none slight diagonally incident lightreduction reduction Haze (change relative perpendicular 0 0 −0.1 −0.1  +0.2 +0.8   −0.1   −0.1 to substrate)/Δ % incident light 45° incident0 0 +0.1 +0.1   +0.3 +1.2   +0.6   +1.0 light Cloudiness upon diagonallynone None none none none None slight slight incident light cloudinesscloudiness Visually detectable external none None none none none stronginterference interference appearance anomalies inter- color + slightcolor + slight ference cloudiness cloudiness color Overall evaluationsFilm adhesion 100/100 100/100 100/100 100/100 5/100 5/100 90/100 90/100Film strength ∘ ∘ ∘ ∘ x ∘ x x Stain resistance ∘ ∘ ∘ ∘ Δ x Δ Δ Opticalproperties ∘ ∘ ∘ ∘ Δ x x x OK OK OK OK durability stain externalexternal NG resistance appearance appearance NG NG NG (Notes) S method:silicate obtained by hydrolysis and condensation of silica. T method:silicate obtained by hydrolysis and condensation of TEOS.

INDUSTRIAL APPLICABILITY

The hydrophilic coating solution of the present invention is aninorganic hydrophilic coating solution, yet can form a film that cansuppress light reflection and coherence, and has a substrate stainresistance function derived from the extremely high hydrophilicity.Moreover, the coating solution is stable in aqueous systems and neutralenvironments, and can form a coating film even by curing at normaltemperature, meaning handling of the solution is safe and easy, duringboth industrial inline production and onsite operations, and therefore astrong and transparent hydrophilic coating can be applied easily,primarily to residential construction materials such as glass andexterior walls.

1. An inorganic hydrophilic coating solution comprising: (a) an aqueoussolution comprising water and an amorphous silicate compound obtained byhydrolyzing and condensing a tetrafunctional silicon compound having apurity of 99.0 mass % or greater in an aqueous medium in the presence ofa basic compound at a temperature within a range from 10 to 170° C., and(b) optionally, not more than 30 mass % of an alcohol, a ketone, asurfactant, or a mixture of two or more thereof, wherein a concentrationof a solid fraction derived from the aqueous solution comprising theamorphous silicate compound is 0.01 to 2.0 mass %, and a pH is 5 to 8.2. The inorganic hydrophilic coating solution of claim 1, wherein acontact angle with water of a coating film obtained by drying and curingthe inorganic hydrophilic coating solution is not more than 20°.
 3. Theinorganic hydrophilic coating solution of claim 1, wherein the basiccompound is an organic base.
 4. The inorganic hydrophilic coatingsolution of claim 1, wherein the basic compound is at least one organicbase selected from the group consisting of an organic ammonium salt, analkylamine, an alkanolamine, and a heterocyclic compound comprisingnitrogen, and an amount added of the basic compound in the hydrolysisand condensation is 100 mol % or more relative to the tetrafunctionalsilicon compound.
 5. The inorganic hydrophilic coating solution of claim1, wherein the tetrafunctional silicon compound is: an amorphous silicahaving an SiO₂ content of 99.0 mass % or greater, a content of each ofNa₂O, K₂O, Fe₂O₃, CaO, SO₃, MgO, and P₂O₅ of 0.1 mass % or less, and aprimary particle size of 500 nm or less, a tetrafunctional siliconalkoxide compound having a purity of 99.0 mass % or greater, atetrafunctional silicon halide compound having a purity of 99.0 mass %or greater, or a combination of two or more thereof.
 6. The inorganichydrophilic coating solution of claim 1, wherein in a spectrum obtainedby performing a ²⁹Si-NMR measurement of the aqueous solution comprisingthe amorphous silicate compound, when a surface area ratio of a peakattributable to a Q_(n) structure, wherein n is an integer of 0 to 4,relative to all peaks is used to calculate a molar ratio R_(n) wherein nis an integer of 0 to 4, of silicon atoms within the Q_(n) structurerelative to all silicon atoms within the aqueous solution comprising theamorphous silicate compound, then the relationships shown below aresatisfied:(R₀+R₁+R₂+R₃)≧90 mol %, R₃≧40 mol %, and R₄≦5 mol %.
 7. The inorganichydrophilic coating solution of claim 1, further comprising: (d) atleast one type of microparticles selected from the group consisting ofmetal oxide particles, metal chalcogenide particles and organometalliccomplex particles, wherein the microparticles have n-type semiconductorproperties and have a primary particle size ofl to 100 nm.
 8. Theinorganic hydrophilic coating solution of claim 7, wherein themicroparticles are at least one type of microparticles selected from thegroup consisting of titanium dioxide particles and tungsten trioxideparticles.
 9. The inorganic hydrophilic coating solution of claim 7,wherein at least one metal selected from the group consisting ofvanadium, manganese, iron, cobalt, nickel, copper, zinc, niobium,molybdenum, ruthenium, rhodium, palladium, silver, tin, tungsten,platinum, and gold, a compound of one of these metals, or a combinationthereof is supported on the microparticles.
 10. An inorganic hydrophiliccoating film obtained by drying and curing the inorganic hydrophiliccoating solution of claim
 1. 11. The hydrophilic coating film of claim10, wherein a contact angle with water is not more than 20° , and acontact angle with water following storage in a dark place for one monthis not more than 20°.
 12. A member comprising a substrate, and thehydrophilic coating film of claim 10 disposed on a surface of thesubstrate.
 13. The member of claim 12, wherein the substrate comprises aglass, a polycarbonate, an acrylic, a polyester, or fluorine.
 14. Themember of claim 12, wherein when the surface on which the hydrophiliccoating film is disposed is observed at any angle from the normaldirection relative to the surface through to the tangential direction ofthe surface, neither interference color nor cloudiness anomalies can bevisually detected.
 15. A cover panel comprising the member of claim 12,wherein the cover panel is suitable for a solar cell module.